In this Age, as is elsewhere told, Sauron arose again in Middle-earth, and grew, and turned back to the evil in which he was nurtured by Morgoth, becoming mighty in his service. Already in the days of Tar-Minastir, the eleventh King of Númenor, he had fortified the land of Mordor and had built there the Tower of Barad-dûr, and thereafter he strove ever for the dominion of Middle-earth, to become a king over all kings and as a god unto Men.

The Silmarillion, Akallabêth

[Sauron] came in secret... to his ancient kingdom of Mordor beyond the Ephel Dúath, the Mountains of Shadow.... There above the valley of Gorgoroth was built his fortress vast and strong, Barad-dûr, the Dark Tower....